The transference of post-dentary jaw elements to the cranium of mammals as auditory ossicles is one of the central topics in evolutionary biology of vertebrates. Homologies of these bones among jawed vertebrates have long been demonstrated by developmental studies, but fossils illuminating this critical transference are sparse and often ambiguous.
Paleontologists from the American Museum of Natural History and the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP), Chinese Academy, announce the discovery of Liaoconodon hui, a complete fossil mammal from the Mesozoic found in China that includes the long-sought transitional middle ear, as apublished in Nature (Vol. 472, pp: 181–185) on 14 April 2011. The specimen shows the bones associated with hearing in mammals -- the malleus, incus, and ectotympanic -- decoupled from the lower jaw, as had been predicted, but were held in place by an ossified cartilage that rested in a groove on the lower jaw. The new study suggests that the middle ear evolved at least twice in mammals, for monotremes and for the marsupial-placental group.
Liaoconodon hui, an Early Cretaceous eutriconodont mammal from the Jehol Biota, Liaoning, China, fills the gap in knowledge between the basal, early mammaliaforms like Morganucodon, where the middle ear bones are part of the mandible and the definitive middle ear of living and fossil mammals. Liaoconodon hui is a medium-sized mammal for the Mesozioc (35.7 cm long from nose to tip of tail, or about 14 inches) and dates from 125 to 122 million years. It is named in part for the bountiful fossil beds in Liaoning, China, where it was found. The species name, hui, honors paleontologist Yaoming Hu who graduated from the American Museum of Natural History-supported doctoral program and recently passed away. The fossil is particularly complete, and its skull was prepared from both dorsal and ventral sides, allowing Meng and colleagues to see that the incus and malleus have detached from the lower jaw to form part of the middle ear. These bones remain linked to the jaw by the ossified Meckel's cartilage that rests in the groove on the lower jaw. The team hypothesizes that in this early mammal, the ear drum was stabilized with the ossified cartilage as a supporting structure.
"People have been looking for this specimen for over 150 years since noticing a puzzling groove on the lower jaw of some early mammals, " said MENG Jin, lead author, curator of the Division of Paleontology the American Museum of Natural History and guest professor of the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing. "Now we have cartilage with ear bones attached, the first clear paleontological evidence showing relationships between the lower jaw and middle ear."
"Before we did not know the detailed morphology of how the bones of the middle ear detached, or the purpose of the ossified cartilage," said MENG. "Liaoconodon hui changes previous interpretations because we now know the detailed morphology of the transitional mammal and can propose that the ossified cartilage is a stabilizer."
In addition to Meng, authors of the paper include WANG Yuanqing and LI Chuankui, both of the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences in Beijing. The research was funded by Major Basic Research Project of the Ministry of Science and Technology, China, the National Science Foundation of China, the Special Fund for Fossil Excavation and Preparation of the Chinese Academy of Sciences, and the National Science Foundation of USA.
The skeleton of the new mammal Liaoconodon hui (dorsal view of the holotype, IVPP V16051, Institute of Vertebrate Paleontology and Paleoanthropology, Beijing). (Image by WANG Yuanqing )